1. Field of the Invention
The present invention relates to apparatus producing a polarized laser beam with high quality spatial characteristics. In particular, the present invention applies to solid state media in the path of a laser beam which exhibit thermally induced stress birefringence for improving beam quality.
2. Description of Related Art
Many solid state lasers exhibit birefringence induced by thermal stress in the lasing medium. The birefringence within the medium has a complex effect on the transverse distribution of power within the beam. In lasers requiring polarized output and control of transverse modes, the induced birefringence places an upper limit on the power at which the medium may be operated in the prior art.
The birefringence phenomenon is especially difficult to eliminate because its distribution follows a cross-section of the lasing medium and the distribution of energy absorbed within that cross-section. Thus, to compensate for induced stress birefringence with wave retardation elements, one must either find elements that match the medium or design the medium to match an available wave retardation optic. In solid state rods, the most common form of solid state media, the birefringence takes on a radial distribution. Because this distribution does not match commonly available wave retardation optics, various special optical elements have been devised to compensate for the induced birefringence with limited success (see Koechner, Applied Optics, Vol. 9, NO. 6, June 1970; see also U.S. Pat. No. 4,408,334, entitled WAVE PLATE FOR CORRECTING THERMALLY-INDUCED STRESS BIREFRINGENCE IN SOLID STATE LASERS; issued Oct. 4, 1983; inventor Lundstrom).
Accordingly, when the power of a solid state laser which exhibits thermally-induced stress birefringence is increased, the magnitude of the birefringence increases within the medium. As the magnitude increases according to the teaching of prior art systems, the spatial quality of the output beam suffers. Thus, a need exists for a high power solid state laser that generates a beam with high spatial quality.
Aside from prior art systems designed to compensate for birefringence, there exists a variety of applications for round laser beams. However, assuring that a given beam is round, in the prior art, is extremely difficult.